The overall objective is to understand the molecular function of the rod cGMP-gated channel. The proposal focuses on studies of the action of APT-cGMP, a photoaffinity analog of the native ligand. Three major issues are to be addressed: (1) the molecular basis for functional heterogeneity in ligand binding sites in the rod channel, specifically, whether the individual alpha and beta subunits of the native channel are subject to independent modulation; (2) how each of the multiple cGMP-binding events on a given channel contribute to activation, specifically, how open states of different conductance arise and whether the channel's beta subunit participates in activation; and (3) how specific amino acids in the channel's cGMP-binding region participate in ligand binding and activation, specifically, how C-8 substituted derivatives of cGMP that are potent channel activators interact with the cGMP binding sites.